1. Field of the Invention
The invention relates to a fuel injection control system and, in particular, to a fuel injection control system for a direct injection-spark ignition engine which learns a fuel injection quantity characteristic of an injector with respect to an injection pulse width.
2. Description of Related Art
Typically, direct injection-spark ignition engines are operative to perform stratified charge combustion by spraying fuel in a compression stroke in a specified engine operating zone, such as a lower engine load and speed zone, with the effect of improving fuel consumption. Such a direct injection-spark ignition engine is equipped with a fuel injector which is pulsed by an electronically controlled fuel injection system (which is hereafter referred to as a fuel injection system for simplicity) to open. The fuel injection system determines an injection pulse width upon which the quantity of fuel delivered by a given injector depends and an injection timing at which the injector is caused to open. The injector is operative to spray fuel according to a given fuel injection quantity characteristic with respect to injection pulse width. In order to eliminate differences in fuel injection quantity characteristics of the individual injectors, the fuel injection quantity characteristic of a given injector is modified or corrected by changing a conversion factor between fuel injection quantity and injection pulse width. Specifically, a fuel injection quantity characteristic between fuel injection quantity and pulse width, shown by means of example in FIG. 12, is different in proportional relationship between the major part (normal injection characteristic zone) A and a minute injection characteristic zone B. This results from an increase in the ratio of time spent on injector valve movement to time for which the injector remains open in the minute injection zone B. Further, shown by means of example in FIG. 13, variation in fuel injection quantity characteristic due to differences of the individual injectors becomes greater with a decrease in the quantity of fuel injection. Accordingly, there is not only a change in the given fuel injection quantity characteristic between the normal injection zone A and the minute injection zone B but also a greater variation in fuel injection quantity characteristic among the individual injectors in the minute injection zone B as compared with the normal injection zone A. In particular, since the direct injection-spark ignition engine often experiences cases where a quantity of fuel required according to a given engine operating condition must be sprayed within a considerably short period of time, it is necessary to equip the engine with injectors with a relatively large ratio of injection (a ratio of a quantity of fuel injection to an open time). However, such an injector is hard to be given a minute injection zone B. In addition, since the direct injection-spark ignition engine increases its combustion efficiency when stratified charge combustion is made by spraying fuel in a compression stroke to raise an air-fuel ratio and the quantity of fuel injection is reduced in consequence, the quantity of fuel injection is reduced to an extent which falls into the minute injection zone B during engine operation with lower engine load, such as during idling. Therefore, in these circumstances, when translating the quantity of fuel injection into an injection pulse using only a conversion factor specified according to the fuel injection quantity characteristic for the normal injection zone A, there occurs a determination of accuracy of the control of fuel injection quantity.
In order to prevent such a determination of accuracy of the control of fuel injection quantity, a fuel injection device for a multi-cylinder internal combustion engine has been proposed in, for example, Japanese Unexamined Patent Publication No. 5-214999 that publication differentiates between a conversion factor for a surge operating zone (which corresponds to the minute injection zone B) and a proportional zone in which the quantity of fuel injection is proportional to a period of time for which the injector remains open (which corresponds to the normal injection zone A) and corrects the conversion factor used in the surge operating zone so as to make output torque equal among cylinders.
While the prior art fuel injection device is designed and adapted to regulate relative variation in output torque among the respective cylinders by correcting the conversion factor so as to make output torque equal among cylinders in the surge operating zone, an error in the absolute quantity of fuel injection is not always eliminated. Further, any such correction of the conversion factor has to be made under such engine operating conditions that the quantity of fuel injection falls within the minute injection zone B, in which a demand for stratified charge combustion by compression stroke injection is made. However, since even a slight aberration of ignition timing causes a great change in cylinder pressure or in difference between cylinder pressure and fuel pressure, which affects the quantity of fuel injection, it is hard to make an accurate correction of the conversion factor.
The conversion factor may be determined and corrected according to fluctuations in fuel injection feedback correction value while the engine operates in an engine operating state in which fuel injection feedback control is accomplished according to an output representative of the air-fuel ratio from an oxygen (O.sub.2) sensor to maintain a stoichiometric air-fuel ratio. Since thermal efficiency is inferior in an engine operating state in which the engine operates in a stoichiometric air-fuel ratio as compared with an engine operating state in which the engine performs lean stratified charge combustion, the quantity of fuel injection for a given engine output torque is larger during the engine operation in the stoichiometric air-fuel ratio than during the engine operation with the lean stratified charge combustion. Therefore, a fuel injection quantity zone (marked "a" in FIG. 12) in which the determining and correcting of a conversion factor is accomplished is differentiated from a fuel injection quantity zone (marked "b" in FIG. 12) in the minute injection zone B for engine operation with stratified charge combustion, which results in an inaccurate correction of conversion factor.